The present invention relates generally to a zoom lens and an imager incorporating it, and more particularly to a large-aperture zoom lens suitable as an interchangeable lens for digital still cameras of the TTL mode or the like, and an imager using the same.
With recent far-reaching spread of digital cameras, the wave of digitalization has surged toward just only the so-called compact type cameras but also single-lens reflex type cameras. In particular, the sale of low-cost products has triggered rapid popularization of digital cameras among common users. With the digital type single-lens reflex cameras, on the other hand, no consistent imager size has been available as yet; various products have been proposed with every size from the so-called Leica size commonly used with silver-halide types to the so-called APS size to the Four-Thirds size put forward as a new standard. Often, however, print size remains invariable even with decreasing imager size, and so image enlargement magnification becomes inevitably high, resulting in the need of enhancing resolution demanded for lenses. In view of the properties of an imager that as the angle of obliquely incident light grows large, light rays are more susceptible to shading, a lot more care must be taken of the angle of exit of light from a lens. In addition, digital cameras often require monitor observations in which remaining chromatic aberrations or decreased edge brightness are noticeable, and so they must have higher performance than ever before. With decreasing imager size, the focal length of a lens decreases in proportion to the imager size upon bringing the taking angle of view in alignment with the Leica size. Although this works for full lens-size reductions, yet there is difficulty in cutting down the back focus. In turn, this leads to the need of increasing the so-called retro ratio, which, combined with the high-performance requirement, offers a new challenge in connection with lens arrangements.
Thus, a new design different from that for the prior art silver-halide purposes is now needed for an interchangeable lens for the digital type single-lens reflex cameras.
As described later, the present invention provides a lens well fit for such a digital type single-lens reflex camera in general, and a medium-telephoto zoom lens in particular. More specifically, the invention provides an unheard-of large-aperture lens that ensures an F-number of 2 all over the zooming zone. According to the zoom lens of the invention, dark subjects such as night views can be taken with no or little blurring or failure or fast shutter operation can be done for subjects in motion, so that new expressions impossible with the prior art can be made.
Referring here to the prior art, for instance, Patent Publications 1 and 2 show medium-telephoto zoom lenses for silver-halide applications. Each has an angle of view of about 34° to about 12°, and basically comprises a first group of positive power, a second group of negative power, a third group of positive power and a fourth group of positive power wherein the first group is divided into a front subgroup of positive power and a rear subgroup of positive power, and focusing is carried out with the rear subgroup. However, both the zoom lenses have an F-number of about 2.8.
A zoom lens designed to achieve F2 all over the zooming zone, for instance, is set forth in Patent Publication 3. This zoom lens is of the five-group zoom type comprising a first group of positive power, a second group of negative power, a third group of positive power, a fourth group of negative power and a fifth group of positive power. However, the zoom ratio is of the order of 2 with aberrations being not that corrected.
Zoom lenses for high-magnification TV cameras are typically shown in Patent Publications 4 and 5. Each is of the four-group zoom type comprising a first group of positive power, a second group of negative power, a third group of positive or negative power and a fourth group of positive power, wherein the first group is divided into a front subgroup of negative power and a rear subgroup of positive power and, upon focusing on a near distance, the front and the rear subgroup move toward the image side and the object side, respectively, in the case of Publication 4, and the front subgroup remains fixed and the rear subgroup moves toward the object side in the case of Publication 5. Still, each zoom lens is less than satisfactory in terms of correction of aberrations, especially chromatic aberrations at the telephoto end.